An active matrix LCD device has the advantages of portability, low power consumption, and low radiation, and has been widely used in various portable information products such as notebooks, personal digital assistants (PDAs), video cameras and the like. Furthermore, the active matrix LCD device is considered by many to have the potential to completely replace CRT (cathode ray tube) monitors and televisions.
FIG. 8 is a circuit diagram of one pixel unit of a typical active matrix LCD, also showing a gate driver and a data driver of the active matrix LCD. The active matrix LCD 100 includes an LCD panel (not shown), the data driver 112, and the gate driver 111. The LCD panel includes a first substrate (not shown), a second substrate (not shown) arranged in a position facing the first substrate, and a liquid crystal layer (not shown) sandwiched between the first substrate and the second substrate.
The first substrate includes a plurality of gate lines 121 that are parallel to each other and that each extend along a first direction, and a plurality of data lines 122 that are parallel to each other and that each extend along a second direction orthogonal to the first direction. The gate lines 121 cross the data lines 122, thereby define a plurality of pixel units 130 (only one shown).
In each pixel unit, a thin film transistor (TFT) 123 is provided in the vicinity of a respective point of intersection of one of the gate lines 121 and one of the data lines 122. The TFT 123 functions as a switching element. A liquid crystal capacitor 127 and a storage capacitor 128 connected in parallel are also provided.
The TFT 123 includes a gate electrode 1231, a source electrode 1232, and a drain electrode 1233. The gate electrode 1231 is connected to a corresponding gate line 121. The source electrode 1232 is connected to a corresponding data line 122. The drain electrode 1233 is connected to the liquid crystal capacitor 127 and the storage capacitor 128.
The liquid crystal capacitor 127 includes a pixel electrode 124, a corresponding common electrode 125 and liquid crystal molecules of the liquid crystal layer sandwiched between the two electrodes 124, 125. The pixel electrode 124 is formed on the first substrate and is connected to the drain electrode 1233 of the TFT 123. The corresponding common electrode 125 is formed on the second substrate.
When the active matrix LCD 100 works, an electric field between the pixel electrode 124 and the common electrode 125 is applied to the liquid crystal molecules of the liquid crystal layer. Light from a light source such as a backlight passes through the second substrate, the liquid crystal layer, and the first substrate. The amount of the light penetrating the substrates is adjusted by controlling the strength of the electric field, in order to obtain a desired optical output for the pixel unit 130.
If an electric field between the pixel electrode 124 and the common electrode 125 continues to be applied to the liquid crystal material in one direction, the liquid crystal material may deteriorate. Therefore, in order to avoid this problem, pixel voltages that are provided to the pixel electrode 124 are switched from a positive value to a negative value with respect to a common voltage of the common electrode 125. This technique is referred to as an inversion drive method.
FIG. 9 is a timing chart illustrating operation of the active matrix LCD 100. In the chart, a Cartesian x-axis (not shown) represents time, and a Cartesian y-axis (not shown) represents voltage. V1g represents a plurality of scanning signals provided by the gate driver 111. V1s represents a plurality of gradation voltages provided by the data driver 112. V1d represents a plurality of pixel voltages of the pixel electrode 124. ΔVg represents an impulse width of each scanning signal Vg, and is equal to a difference between a gate-on signal Von and a gate-off signal Voff. V1com represents a common voltage of the common electrode 125 provided by an external circuit (not shown). ΔV represents a voltage distortion related to the pixel voltage V1d.
When a gate-on voltage Von is provided to the gate electrode 1231 of the TFT 123 via the gate line 121, the TFT 123 connected to the gate line 121 turns on. At the same time, a gradation voltage V1s generated by the data driver 112 is provided to the pixel electrode 124 via the data line 122 and the activated TFT 123 in series. The potentials of the common electrodes 125 are set at a uniform potential V1com. Accordingly, the liquid crystal capacitor 127 and the storage capacitor 128 connected in parallel are charged to obtain a voltage difference between the gradation voltage V1s and the common voltage V1com. Therefore, an electric field is generated due to the voltage difference between the pixel electrode 124 and the common electrode 125. The electric field is used to control the amount of light transmission of the corresponding pixel unit 130.
When a gate-off voltage V1off is provided to the gate electrode 1231 of the TFT 123 via the gate line 121, the TFT 123 turns off. The gradation voltage V1s applied to the liquid crystal capacitor 127 while the TFT 123 is turned on should be maintained as the pixel voltage V1d by the liquid crystal capacitor 127 and the storage capacitor 128 after the TFT 123 turns off. However, due to a parasitic capacitance Cgd (not shown) between the gate electrode 1231 and the drain electrode 1233 of the TFT 123, the pixel voltage V1d of the pixel electrode 124 is distorted when the TFT 123 turns off. This kind of voltage distortion ΔV is known as a kick-back voltage, and the kick-back voltage is obtained by following formula:
                              Δ          ⁢                                          ⁢          V                =                                            C              gd                                                      C                gd                            +                              C                lc                                              ×          Δ          ⁢                                          ⁢                      V            g                                              (        1        )            The voltage distortion ΔV always tends to reduce the pixel voltage Vd regardless of the polarity of the data voltage, as shown in FIG. 9.
The pixel voltage V1d of the pixel electrode 124 after the TFT 123 turns off is less than the gradation voltage V1s applied to the pixel electrode 124 before the TFT 123 turns off. Accordingly, the electric field used to control the amount of light transmission of the corresponding pixel unit 130 is decreased when the TFT 123 turns off. Therefore, a light transmission of the corresponding pixel unit 130 when the TFT 123 turns on is greater than a light transmission of the corresponding pixel unit 130 when the TFT 123 turns off. As a result, the so-called flicker phenomena appears on a display screen of the active matrix LCD 100.
What is needed, therefore, is an active matrix LCD that can overcome the above-described problems. What is also needed is a related method for driving such kind of active matrix LCD.